Last week, the New York Times fitness maven Gretchen Reynolds published an article titled “Slow Runners Come Out Ahead,” where she relayed the findings of a paper published this month in the Journal of the American College of Cardiology comparing the mortality rates of joggers and sedentary non-joggers. The former group was subcategorized into light, moderate, and strenuous; hence the title of the JACC paper: “Dose of Jogging and Long-Term Mortality.”

While the study came to the unsurprising conclusion that light and moderate joggers had lower mortality rates than the inactive participants, the shocker was that those who fell into the strenuous category had nothing, longevity-wise, on couch-bound folks. As Reynolds writes:

Plodding joggers tended to live longer than those who ran faster. In fact, the people who jogged most often and at the fastest pace—who were, in effect, runners rather than joggers—did not enjoy much benefit in terms of mortality. In fact, their lifespans tended to be about the same as those who did not exercise at all.

If anything is likely to catch fire on the Internet, it’s the vindication of one’s bad habits. And so last week we were treated to a flurry of headlines announcing the findings of the JACC study, which make the title of Reynolds’s article seem coy in comparison:

You don’t need to be the most skeptical person in the universe to suspect that something may be amiss here. (What, after all, could possibly be as deadly as sitting on the couch?) Interestingly (or ironically?) one of the most pointed criticisms of the study’s “findings” was another Times piece, published the day after the first one. In “No, More Running Probably Isn’t Bad for You,” Justin Wolfers provides a useful reminder that whenever we get bombarded with glib headlines trumpeting the results of the latest study we’d be well advised to look closely at what we can actually glean from the data.

In the case of “Dose of Jogging and Long-Term Mortality,” the answer is: not much. As Wolfers points out, the pool of test subjects for the study was so small that the results don’t really tell us anything.

The researchers asked Danish runners about the speed, frequency, and duration of their workouts, categorizing 878 of them as light, moderate, or strenuous joggers. Ten years later, the researchers checked government records to see how many of them had died. Happily, only 17 had. While this was good news for the surviving runners, it was bad news for the researchers, because 17 was clearly too few deaths to discern whether the risk of death was related to running intensity.

Out of the 878 active subjects, only 40 self-identified as strenuous joggers, of which a grand total of two died over the long-term testing period. This was compared with seven deaths out 576 light joggers and eight deaths out of 262 moderate joggers. Forget that this isn’t enough data to make any grand conclusions about the effect of exercise intensity on projected lifespan—the study didn’t even take the cause of death into consideration.

In other words, if an American version of the study had been conducted during the high-mileage heyday of the ‘70s, and the strenuous group of subjects had included a young Oregonian named Steve Prefontaine, his tragic car crash would have contributed to the statistical “evidence” that too much running is bad for your health.

In fairness to the authors of the JACC study, and Gretchen Reynolds, both acknowledge that further studies are needed to determine whether your Sunday 20-miler might actually be cancelling out the positive effects of running. That’s putting it rather mildly. While it seems logical that there is probably such a thing as too much running, we still have no idea how much that might be. (A useful reference here is an extensive Runner’s World article by Alex Hutchinson.) And, as Justin Wolfers writes, “The study doesn’t change what the weight of the evidence shows: Most Americans need to worry about exercising too little, not too much.”

So keep running ultras if that’s what makes you happy. And if you’re worried that a future, more meticulous study might one day make a stronger case that those excess miles are burning your candle at both ends, bear in mind the words of German poet and novelist Erich Kästner, who once wrote: “Leben ist immer Lebensgefährlich.” Life is always life-threatening.

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